"Scan test computer, found trouble codes #P0133 and #P1131. Test and confirm bad oxygen sensor. Replace front O2 sensor"
He changed the Bank 1 O2 sensor the first time as the sensor was tested as bad.
"Computer had code P0133-B1 S1 slow response. P0131-B1 S1 lean. P1151-B2 S1 lean. Tested oxygen sensors. Found wrong sensor was replaced. Replaced correct faulty oxygen sensor. Cleared codes. All parameters are normal now. Road tested OK."
Maybe your Bank 2 O2 sensor was also about to go bad the first time.
It is good that you have a honest mechanic and that you trust will do the right thing.
Note that there could be many reasons for lean codes. Here is a good article to read.
http://www.v8sho.com/SHO/TSB0197HO2SServiceTips.htm
